1. Field of the Invention
This invention relates to radiation curable aqueous compositions and printing inks for the manufacture of packaging materials. More particularly, this invention relates to radiation curable compositions and printing inks for the manufacture of low odor food packaging materials which have low levels of extractable components.
2. Description of Related Art
Energy curable, low viscosity inks and coatings are typically composed of mixtures of acrylated oligomers and monomers. Typically monomers are used to control viscosity of ink or coating formulations for flexographic, gravure, roller and tower printing and coating applications. However, diluent monomers do not react completely during polymerization upon exposure to ultraviolet (UV) or electron beam (EB) radiation. Such unreacted monomers remain as residual components in the dried printing ink or coating films and are subject to migration by absorption as well as surface contact. This migration of residual components leads to a host of problems, particularly for printing or coating xe2x80x9codorxe2x80x9d and xe2x80x9coff-tastexe2x80x9d sensitive packaging for packages such as containers for food, beverages, tobacco, perfume, etc., and for such applications which require negligible amounts of extractables from cured printing inks or coatings such as pharmaceutical and health care packaging. In addition, sometimes solvents are employed to achieve a coating of lower viscosity.
An example of a solvent based coating is described in U.S. Pat. No. 5,824,717, Merill et al., which discloses peroxide and radiation (energy) curable compositions containing isobutylene copolymers having acrylate functionality, and optionally a filler. The disclosed copolymers are acrylate modified copolymers of an iso-olefin of 4 to 7 carbon atoms and para-alkylstyrene co-monomers. Merrill discloses that the percentage of extractables from the cured composition is negligible, and that the cured composition is suitable for use in the manufacture of a variety of high purity rubber goods used in the pharmaceutical and health care industries. Merrill further discloses that the compositions may be used as condenser packings, food contact material, wire cable insulation materials, and in the manufacture of high purity hoses. Merrill discloses that coatings are prepared by dissolving the copolymer in toluene as the primary solvent.
Problems resulting from odor, off-taste and residual extractables with currently available UV/EB printing inks and coatings has kept energy curable products at bay from the high volume packaging market, which still is largely served by conventional solvent or water based flexo printing inks and coatings which require the removal of the solvent or water before curing. Acrylated oligomers typically have viscosities, which are too high to be used per se (i.e., without a monomer diluent) for making low viscosity coatings and printing a especially inks.
The use of water as a diluent for mixtures of UV/EB curable acrylated oligomers is disclosed, however, in U.S. Pat. No. 6,011,078 for application in wood and floor coatings. The formulations are dispersions or emulsions, which require prior evaporation or imbition of water on non-absorbent substrates before exposure to light.
There continues to be a need for single fluid, monomer and solvent free UV/EB curable aqueous printing ink and coating formulations, which produce cured films having insignificant odor, off-taste, and/or extractable components.
The invention is a method for producing a low-extractable film (i.e., printing ink film or coating) comprising the steps of:
(a) providing an actinic radiation curable homogeneous aqueous composition having a water soluble compound which contains at least one alpha, beta-ethylenically unsaturated, radiation polymerizable group; and water; and
(b) applying said homogeneous aqueous composition onto a surface; and
(c) irradiating the surface with actinic radiation in the presence of water to form a cured film; wherein less than 50 ppb of uncured residue is extractable from the cured film when said film is immersed and heated in 10 ml of a simulant liquid per square inch of cured film.
A further embodiment of this invention is an improved actinic radiation curable homogeneous aqueous composition having a water soluble compound which contains at least one alpha, beta-ethylenically unsaturated, radiation polymerizable group; and water; wherein the improvement comprises the requirement that when a surface is coated with the composition and exposed to actinic radiation in the presence of water, a cured film is formed wherein less than 50 ppb of uncured residue is extractable from the cured film when immersed and heated in 10 ml of a simulant liquid per square inch of cured film, Preferably, the water soluble compound is a water soluble oligomer containing two or more acrylic groups.
A still further embodiment of this invention is a packaging material comprising a substrate and a cured film adhered to the surface of the substrate, wherein: the cured film is derived by providing a homogeneous aqueous composition consisting essentially of a water soluble oligomer containing two or more acrylic groups; and water and curing the homogeneous aqueous composition by actinic radiation in the presence of water such that less than 50 ppb of oligomer residue is extractable from the cured film when it is immersed and heated in 10 ml of a simulant liquid per square inch of cured film.